stack - (Optional) The application stack is what platform to run the application
in.

buildpacks - (Optional) Buildpack names or URLs for the application.
Buildpacks configured externally won't be altered if this is not present.

config_vars1 - (Optional) Configuration variables for the application.
The config variables in this map are not the final set of configuration
variables, but rather variables you want present. That is, other
configuration variables set externally won't be removed by Terraform
if they aren't present in this list.

sensitive_config_vars1 - (Optional) This argument is the same as config_vars.
The main difference between the two is when sensitive_config_vars outputs
are displayed on-screen following a terraform apply or terraform refresh,
they are redacted, with displayed in place of their value.
It is recommended to put private keys, passwords, etc in this argument.

Deleting an entire config_vars or sensitive_config_vars map from a heroku_app
configuration will not actually remove the vars on the remote resource. To remove an existing variable,
leave these attribute maps in-place and delete only its entries from the map. Once these attributes are
empty, the map itself may be deleted from the configuration. Otherwise if one deletes the map with existing
entries, the config vars will not be deleted from the remote resource.

This is especially important if you are migrating all config_vars to sensitive_config_vars or migrating
config vars to heroku_app_config_association resource.